In the above example, "Subject" is the field name and the portion that
begins with "=?ISO-8859-1?..." is the field value.

Parameters

field_name

The field name.

field_value

The field value.

preferences

You can control the behaviour of iconv_mime_encode()
by specifying an associative array that contains configuration items
to the optional third parameter preferences.
The items supported by iconv_mime_encode() are
listed below. Note that item names are treated case-sensitive.

Specifies the character set in which the first parameter
field_name and the second parameter
field_value are presented. If not given,
iconv_mime_encode() assumes those parameters
are presented to it in the
iconv.internal_encoding
ini setting.

Specifies the maximum length of the header lines. The resulting
header is "folded" to a set of multiple lines in case
the resulting header field would be longer than the value of this
parameter, according to
» RFC2822 - Internet Message Format.
If not given, the length will be limited to 76 characters.

Specifies the sequence of characters to append to each line
as an end-of-line sign when "folding" is performed on a long header
field. If not given, this defaults to "\r\n"
(CRLF). Note that
this parameter is always treated as an ASCII string regardless
of the value of input-charset.

\r\n

\n

Return Values

Returns an encoded MIME field on success,
or FALSE if an error occurs during the encoding.

As a workaround, you could explode() the value on spaces and encode each word separately. Then remove the "Subject: " in front of the resulting strings and join() them with "\r\n " (don't forget the SPACE after the \n) as separator.